KATHMANDU, Nepal (CNN) -- Two Communist Party workers and a candidate for a small leftist political party were killed less than a month before a historic vote in Nepal, police said Wednesday.

Family members carry the body of Kamal Prasad Adhikari at his cremation procession Wednesday.

Gunmen raided the home of National People's Front candidate Kamal Prasad Adhikari, shooting him four times Tuesday night after he refused to come out of the house in the southeastern town of Nepalganj, police said. He died in a hospital Wednesday morning.

The killing came after the European Union and the United Nations expressed concerns about pre-election violence in Nepal.

The election will set the makeup of Nepal's 601-member constituent assembly, which will decide the fate of the country's monarchy and prepare a new constitution.

The 239-year-old monarchy has been virtually suspended since King Gyanendra was forced to give up powers in April 2006 after a popular uprising against his direct rule.

The U.N. Mission in Nepal, which is helping the country's effort to hold fair elections, expressed its "deep concern" over Adhikari's killing.

"Violence and threats against candidates represent a serious obstacle to the creation of a free and fair atmosphere for the election, and all efforts must be made to bring those responsible to justice," the mission said in a statement.

A police official said the killing is under investigation. But according to local reports, Janatantrik Terai Mukti Morcha claimed responsibility for the attack. It is one of the largest and oldest armed groups pushing for the rights of the ethnic Madhesi people in southern Nepal.

Adhikari was to represent the National People's Front, also known as Rastriya Jana Morcha -- a splinter group of the Communist Party of Nepal -- in the April 10 vote.

Also Tuesday, two Communist Party of Nepal workers were shot dead in separate incidents in remote parts of western Nepal, police said.

The shootings in the western Rolpa district are under investigation, but the Communist Party -- made up of former Maoist rebels -- blamed the attacks on "forces close to the palace, the palace and foreign powers."

Nepal signed a peace deal with Maoist rebels in November 2006, ending a decade-long conflict in which about 13,000 people were killed. The former rebels joined parliament and the interim government early last year as the Communist Party of Nepal.

Tensions among the Madhesi people, who make up nearly half of Nepal's population, began simmering after the agreement with the Maoists. Groups representing the Madhesi said they felt marginalized and feared that they would not have a place in a future government.

Last month, Nepal's government signed a deal with the United Democratic Madhesi Front, paving the way for the elections and ending a crippling strike.

After visiting the country last year, U.N. High Commissioner for Human Rights Louise Arbour described the tensions between the Madhesi people and the government as one of many "long-simmering grievances now surfacing" as a result of the peace deal with the Maoists.

"What is happening in [southern Nepal] was inevitable as the space for democracy opens up the possibility of airing grievances increases," she said. "Democracy is a lot more chaotic in that sense than autocratic rule."E-mail to a friend